1. Field of the Invention
The present invention relates to heat engine parts made of an alloy having good mechanical strength and resistance to high temperatures, and comprising a protective coating, and relates especially to turbo-machine parts made of superalloy, particularly if nickel-based, having a protective coating for protection against corrosion and oxidation. The invention also relates to a method of forming the protective coating on such parts.
2. Summary of the prior art
The search for high performance in the development of turbo-machines, particularly for aeronautical applications, has led to ever increasing operating temperatures, while rationalization of the use of equipment makes it necessary to prolong the life of the machine parts. The result of this is the adoption of numerous solutions relating to protective coatings for providing turbo-machine parts subjected to high temperatures with protection against oxidation and/or corrosion.
U.S. Pat. No. 4,328,285 discloses the protection of superalloy gas turbine parts by a metallic undercoat having a composition of the M, Cr, Al, Y type, where M denotes Fe, Ni, Co or a mixture of these metals, applied by plasma spraying, followed by a ceramic-based coating comprising zirconium oxide and at least 15% by weight of cerium oxide, also formed by plasma spraying.
U.S. Pat. No. 4,248,940 discloses another example of a heat barrier coating for superalloy parts, the coating being formed by plasma spraying a mixture of powders comprising a bonding material of the M, Cr, Al, Y type, where M denotes Fe, Ni, Co or a mixture thereof, and a ceramic type material based on zirconium oxide stabilized by another oxide, the coating having an increasing percentage of ceramic in the direction away from the substrate.
However, no previously known solution is entirely satisfactory in relation to the particular conditions of use and taking into account the operating requirements and the improvement of the properties of thermal insulation and of resistance to combined oxidation and corrosion agents of various kinds. A particularly noticeable phenomenon has been observed which may be described as the development and the propagation of cracks or fissures under the action of stresses which develop in the coating and which are of thermal origin in particular.
Other heat engines, particularly diesel engines, also have parts which have been provided with a protective coating for the improvement of operating performance.
It is an object of the invention, therefore, to provide heat engine parts with an improved protective coating structure, and in particular a structure capable of modifying the manner of rupture observed in the coating under critical conditions of operation of the coated parts.